The essence of an Adaptive Cruise Control (ACC) or Forward Collision Warning (FCW) system is the forward ranging sensor used to detect target vehicles or obstacles in the vehicle's lane or path. The primary sensor used for this purpose has been millimeter wave, frequency modulated continuous wave (FMCW) radar. These sensors are derived from a military background and are capable of accurate measurements of range, range rate, and azimuth to multiple targets within the radar field of view. However, a key component missing from this information is the ability to interpret the acquired target's position in the context of the roadway. Early radar systems made no attempt at discriminating potential targets based on the road geometry. The assumption was the primary target was that which was most aligned with the bore sight of the vehicle. This works fine on long straight roads. However, during curves and lane change maneuvers, the system would often lock onto incorrect targets. Once the system had acquired a target, the advanced target tracking would allow the radar to track the vehicle through curves. However, the acquisition of new targets in these transition maneuvers proved to be a fundamental problem with this most basic sensing system.
Other known systems include a vehicle dynamic based system, such as the system disclosed in U.S. Pat. No. 5,467,283 issued to Butsuen et al., and a far field vision system, such as the system disclosed in U.S. Pat. No. 5,343,206 issued to Ansaldi et al. The vehicle dynamic based system utilizes a lane estimation algorithm based on the dynamic state of the vehicle as measured by an on-board yaw rate gyro. This system assumes that the vehicle is precisely tracking the curvature of the road and, therefore, the measured yaw rate of the vehicle is proportional to the instantaneous curvature of the road. This system also assumes that the driver precisely regulates to the center of the lane with zero heading angle. In the mean, these assumptions must be true if the vehicle is to stay on the road. However, there will definitely be transient situations when these assumptions are not valid.
Far field vision systems utilize a camera to pick up an image of the road ahead of the vehicle. The camera picks up and processes white lines drawn on the road surface ahead of the vehicle to determine the road geometry. These systems, however, are subject to lane mark visibility due to weather (e.g., snow) and traffic conditions (e.g., trucks having large white trailers), flat earth model assumptions and camera motion.
Thus, room for improvement exists in future systems, such as FCW where the system will be called upon to make highly autonomous decisions about the nature of obstacles in the radar field of view.